Recording material

ABSTRACT

A recording material wherein a color forming system comprising (a) a fluorene compound as a dye precursor, (b) a color developer, (c) an isocyanate compound, and (d) in imido compound, gives a developed color image having an absorption at near infrared portion and excellent image storability.

This is a continuation of Application No. 07/122,191, filed Nov. 18,1987.

BACKGROUND OF THE INVENTION

This invention relates to a two-component color forming type recordingmaterial having absorptions particularly at near infrared portion andgiving developed color images excellent in storability.

As recording materials applying a two-component color forming systemcomprising a colorless or light-colored dye precursor and a colordeveloper which can form a color in contact with the dye precursor,there are well known pressure-sensitive recording materials, andheat-sensitive recording materials. Demand for these materials increasesyear after year. Further, electric current-passing heat-sensitiverecording materials, light-sensitive recording materials, heat transferrecording materials are also developed recently.

Heat-sensitive recording materials generally comprise a support andformed thereon a color forming layer containing as major components anelectron donative colorless or light-colored dye precursor and anelectron-accepting color developer. Upon heating with a thermal head, athermal pen, a laser light, or the like, the dye precursor and the colordeveloper react instantly to give a recording image (e.g. JapanesePatent Examined Publication Nos. 43-4160 and 45-14039). Since theseheat-sensitive recording materials have advantages in that recording canbe obtained by using a relatively simple apparatus, the maintenance iseasy and no noise is produced, they are widely used in recorders formeasuring, facsimile machines, printers, computer terminals, labels,automatic ticket vending machines, etc.

Particularly, heat-sensitive recording labels are used as bar codelabels for point of sales (POS) system, price labels, delivery andskipping labels and the like, wherein adhering is necessary afterprinting. The POS system remarkably spreads in large-scale stores suchas supermarkets and department stores as well as chain stores of specialstores, restaurants, etc. In order to grasp the needs of consumersclearly and to plan reasonable business strategy, it is an object tototalize and analyze sales information of goods. In order to attain suchan object, methods for reading bar codes and OCR letters adhered togoods sold in stores with a scanner optically have been developedrapidly. As a method for printing bar codes, etc., there is widely useda heat-sensitive recording method which has advantages in thatminiaturization and simplification of the machine is possible, speed upof recording is possible, there is no contamination of hands and goodsby the ink, and there is no fear of blur of ink.

As a method for reading developed color images such as bar codes, etc.,there has been used a He.Ne laser light having a red color at awavelength of 633 nm. But recently, the use of a semiconductor laser iswidely spreading. This is because it has many advantages in that themodulation can be made directly by an electric current, miniaturizationis possible, the use of semiconductor laser is easy with a low cost,erroneous operation due to contamination is rare because the oscillationwavelength is in the near infrared portion of 700 to 1500 nm. Therefore,a demand for reading developed color images in heat-sensitive recordinglabels by using the semiconductor laser increases. According to knowntwo-component type heat-sensitive recording materials comprising a dyeprecursor and a color developer, the absorption wavelength of developedcolor images is, even in a black color-forming system, at a longerwavelength side of 550 to 620 nm which range can be read by a He.Nelaser light but cannot be read by using a laser light having awavelength at the near infrared portion. Some of the present inventorshave provided heat-sensitive recording materials readable by a laserlight at the near infrared portion (e.g. Japanese Patent UnexaminedPublication Nos. 61-175077, 61-228985, 61 -228986, etc.), but they arestill insufficient in storability of developed color images,particularly insufficient in light resistance.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a recording material givinga developed color image having an absorption at near infrared portionand excellent image storability, particularly excellent in lightresistance.

This invention provides a recording material which comprises a colorforming layer containing a color forming system comprising

(A) (a) a colorless or light-colored dye precursor represented by theformula: ##STR1## wherein R₁ through R₆ are independently a lower alkylgroup, (b) a color developer which can form a color by reacting with thedye precursor, and

(B) (c) an isocyanate compound having aromaticity, and

(d) an imino compound having at least one >C═NH group,

said color forming layer being carried on a support or at least twodifferent supports.

DETAILED DESCRIPTION OF THE INVENTION

The colorless or light-colored dye precursor (a) used in this inventionis a fluorene compound represented by the formula: ##STR2## wherein R₁through R₆ are independently a lower alkyl group preferably having 1 to6 carbon atoms. The fluorene compound of the formula (I) is a colorlessor light-colored crystal and has no absorption at the near infraredportion. But when it is reacted with a color developer, a color fromblue to green is formed and the developed color image has an absorptionat the near infrared portion.

Examples of the fluorene compound of the formula (I) aRe3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylaminophthalide),3-diethylamino-6-dimethylaminofluorene-9-spiro-3'-(6'-dimethylaminophthalide),3,6-bis(diethylamino)fluorene-9-spiro-3'-(6'-dimethylaminophthalide),3-dibutylamino-6-dimethylaminofluorene-9-spiro-3'-(6'-dimethylaminophthalide),3-dibutylamino-6-diethylaminofluorene-9-spiro-3'-(6'-dimethylaminophthalide),3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-diethylaminophthalide),3-diethylamino-6-dimethylaminofluorene-9-spiro-3'-(6'-diethylaminophthalide),3-dibutylamino-6-dimethylaminofluorene-9-spiro-3'-(6'-diethylaminophthalide),3,6-bis(diethylamino)fluorene-9-spiro-3'-(6'-diethylaminophthalide),3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dibutylaminophthalide),3-dibutylamino-6-diethylaminofluorene-9-spiro-3'-(6'-diethylaminophthalide),3-diethylamino-6-dimethylaminofluorene-9-spiro-3'-(6'-dibutylaminophthalide),etc. These fluorene compounds can be used alone or as a mixture thereof.

The color developer (b) which can form a color by reacting with thefluorene compound with heating is an electron-accepting compound such asphenol derivatives, aromatic carboxylic acid derivatives, etc.

Examples of the phenol derivatives are phenol, p-t-butylphenol,p-phenylphenol, 1-naphthol, 2-naphthol, p-hydroxyacetophenone,2,2'-dihydroxybiphenyl, 4,4'-isopropylidenediphenol,4,4'-isopropylidene-bis(2-t-butylphenol),4,4'-isopropylidenebis(2-chlorophenol), 4,4'-cyclohexylidenediphenol,2,2-bis(4-hydroxyphenyl)-butane, 2,2-bis(4-hydroxyphenyl)pentane,2,2-bis(4-hydroxyphenyl)hexane, methyl diphenolacetate,1,7-bis-(4-hydroxyphenylthio)-3,5-dioxiheptane, novolac type phenolresin, etc.

Examples of the aromatic carboxylic acid derivatives are benzoic acid,p-t-butyl benzoate, p-hydroxybenzoic acid, methyl p-hydroxybenzoate,isopropyl p-hydroxybenzoate, benzyl p-hydroxybenzoate, lauryl gallate,stearyl gallate, salicylanilide, 5-chlorosalicylanilide, a metal salt(e.g. Zn) of 5-t-butyl salicylate, a metal salt (e.g. Zn) ofhydroxynaphthoic acid, etc.

It is also possible to use sulfone compounds and sulfide compoundspreferably. Examples of the sulfone and sulfide compounds arebis(4-hydroxyphenyl)-sulfone, 4-hydroxy-4'-methyldiphenylsulfone,4-hydroxy-4'-isopropyloxydiphenylsulfone,bis(3-allyl-4-hydroxyphenyl)sulfone,bis(3-methyl-4-hydroxyphenyl)sulfone,bis(3-butyl-4-hydroxyphenyl)sulfone,bis(3,5-diallyl-4-hydroxyphenyl)sulfone,bis(3-chloro-4-hydroxyphenyl)-sulfone,3-allyl-4,4'-dihydroxydiphenylsulfone,bis(2-ethyl-4-hydroxyphenyl)sulfone,3-isopropyl-4,4'-dihydroxydiphenylsulfone,bis(2,5-dimethyl-4-hydroxyphenyl)sulfone,bis(2,5-dichloro-4-hydroxyphenyl)sulfone,4,4'-thiobis(2,5-dichlorophenol), 4,4'-thiodiphenol,2,2'-thiobis(4-chlorophenol), 2,2'-thiobis(4,6-dichlorophenol),4,4'-thiobis(2,6-dimethylphenol), 4,4'-thiobis(2,5-dimethylphenol),4,4'-thiobis(2-isopropyl-5-methylphenol),4,4'-thiobis(2-cyclohexylphenol), 4,4'-thiobis(2-isopropylphenol),4,4'-thiobis(3-ethylphenol), 4,4'-thiobis(2-t-butyl-5-methylphenol),etc.

The isocyanate compound having aromaticity (c) includes aromaticisocyanates and heterocyclic isocyanates which are solid at roomtemperature and colorless or light-colored.

Examples of the isocyanate compounds are 2,6-dichlorophenyl isocyanate,p-chlorophenyl isocyanate, 1,3-phenylene diisocyanate, 1,4-phenylenediisocyanate, 1,3-dimethylbenzene 4,6-diisocyanate, 1,4-dimethylbenzene2,5-diisocyanate, 1-methoxybenzene 2,4-diisocyanate, 1-methoxybenzene2,5-diisocyanate, 1-ethoxybenzene 2,4-diisocyanate, 2,5-dimethoxybenzene1,4-diisocyanate, 2,5-diethoxybenzene 1,4-diisocyanate,2,5-dibutoxybenzene 1,4-diisocyanate, azobenzene 4,4'-diisocyanate,diphenyl ether 4,4'-diisocyanate, naphthalene 1,4-diisocyanate,naphthalene 1,5-diisocyanate, naphthalene 2,6-diisocyanate, naphthalene2,7-diisocyanate, 3,3'-dimethylbiphenyl 4,4'-diisocyanate,3,3'-dimethoxybiphenyl 4,4'-diisocyanate, diphenylmethane4,4'-diisocyanate, diphenyldimethylmethane 4,4'-diisocyanate,benzophenone 3,3'-diisocyanate fluorene 2,7-diisocyanate, anthraquinone2,6-diisocyanate, 9-ethylcarbazole 3,6-diisocyanate, pyrene3,8-diisocyanate, naphthalene 1,3,7-triisocyanate, biphenyl2,4,4'-triisocyanate, 4,4',4"-triisocyanate-2,5-dimethoxytriphenylamine,p-dimethylaminophenyl isocyanate, tris(4-phenyl isocyanate)thiophosphate, etc. These isocyanate compounds can be used alone or as amixture thereof.

If necessary, these isocyanate compounds can be used in the form ofso-called block isocyanates, which are adducts of phenols, lactams,oximes, and the like. It is also possible to use dimers of diisocyanatessuch as a dimer of 1-methylbenzene 2,4-diisocyanate, isocyanurates whichare trimers of diisocyanates and various polyisocyanates which areadducts with various polyols.

The imino compound having at least one >C═NH group (d) is a compoundrepresented by the formula:

    ΦC═NH                                              (II)

wherein Φ is an aromatic compound residue which can form a conjugatedsystem with the neighboring C═N, and is solid at room temperature andcolorless or light-colored.

Examples of the imino compound are 3-iminoisoindolin-1-one,3-imino-4,5,6,7-tetrachloroisoindolin-1-one,3-imino-4,5,6,7-tetrabromoisoindolin-1-one,3-imino-4,5,6,7-tetrafluoroisoindolin-1-one,3-imino-5,6-dichloroisoindolin-1-one,3-imino-4,5,7-trichloro-6-methoxy-isoindolin-1-one,3-imino-4,5,7-trichloro-6-methylmercapto-isoindolin-1-one,3-imino-6-nitroiso-indolin-1-one, 3-imino-isoindolin-1-spiro-dioxolane,1,1-dimethoxy-3-imino-isoindoline,1,1-diethoxy-3-imino-4,5,6,7-tetrachloroisoindoline,1-ethoxy-3-iminoisoindoline, 1,3-diiminoisoindoline,1,3-diimino-4,5,6,7-tetrachloroisoindoline,1,3-diimino-6-methoxyisoindoline, 1,3-diimino-6-cyanoisoindoline,1,3-diimino-4,7-dithia-5,5,6,6-tetrahydroisoindolone,7-amino-2,3-dimethyl-5-oxopyrro[3,4b]pyrazine,7-amino-2,3-diphenyl-5-oxopyrro[3,4b]pyrazine, 1-iminonaphthalic acidimide, 1-iminodiphenic acid imide, 1-phenylimino-3-iminoisoindoline,1-(3'-chlorophenylimino)-3-iminoisoindoline,1-(2',5'-dichlorophenylimino)-3-iminoisoindoline,1-(2',4',5'-trichlorophenylimino)-3-iminoisoindoline,1-(2'-cyano-4'-nitrophenylimino)-3-iminoisoindoline,1-(2'-chloro-5'-cyanophenylimino)-3-iminoisoindoline,1-(2',6'-dichloro-4'-nitrophenylimino)-3-iminoisoindoline,1-(2',5'-dimethoxyphenylimino)-3-iminoisoindoline,1-(2',5'-diethoxyphenylimino)-3-iminoisoindoline,1-(2'-methyl-4'-nitrophenylimino)-3-iminoisoindoline,1-(5'-chloro-2'-phenoxyphenylimino)-3-iminoisoindoline,1-(4'-N,N-dimethylaminophenylimino)-3-iminoisoindoline,1-(3',N,N-dimethylamino-4'-methoxyphenylimino)-3-iminoisoindoline,1-(2'-methoxy-5'-N-phenylcarbamoylphenylimino)-3-iminoisoindoline,1-(2'-chloro-5'-trifluoromethylphenylimino)-3-iminoisoindoline,1-(5',6'-dichlorobenzothiazolyl-2'-imino)-3-iminoisoindoline, 1-(6'-methylbenzothiazolyl-2'-imino)-3-iminoisoindoline,1-(4'-phenylaminophenylimino)-3-iminoisoindoline,1-(p-phenylazophenylimino)-3-iminoisoindoline,1-(naphthyl-1'-imino)-3-iminoisoindoline,1-(anthraquinone-1'-imino)-3-iminoisoindoline,1-(5'-chloroanthraquinone-1'-imino)-3-iminoisoindoline,1-(N-ethylcarbazolyl-3'-imino)-3-iminoisoindoline,1-(naphthoquinone-1'-imino)-3-iminoisoindoline,1-(pyridyl-4'-imino)-3-iminoisoindoline,1-(benzimidazolone-6'-imino)-3-iminoisoindoline,1-(1'-methylbenzimidazolone-6'-imino)-3-iminoisoindoline,1-(7'-chlorobenzimidazolone-5'-imino)-3-iminoisoindoline,1-(benzimidazolyl-2'-imino)-3-iminoisoindoline,1-(benzimidazolyl-2'-imino) -3-imino-4,5,6,7-tetrachloisoindoline,1-(2',4'-dinitrophenylhydrazone)-3-iminoisoindoline,1-(indazolyl-3'-imino)-3-iminoisoindoline,1-(indazolyl-3'-imino)-3-imino-4,5,6,7-tetrabromoisoindoline,1-(indazolyl-3'-imino)-3-imino-4,5,6,7-tetrafluoroisoindoline,1-(benzimidazolyl-2'-imino)-3-imino-4,7-dithiatetrahydroisoindoline,1-(4',5'-dicyanoimidazolyl-2'-imino)-3-imino-5,6-dimethyl-4,7pyraziisoindoline,1-(cyanobenzoylmethylene)-3-iminoisoindoline,1-(cyanocarbonamidomethylene)-3-iminoisoindoline,1-(cyanocarbomethoxymethylene)-3-iminoisoindoline,1-(cyanocarboethoxymethylene)-3-iminoisoindoline,1-(cyano-N-phenylcarbamoylmethylene)-3-iminoisoindoline,1l-(cyano-N-phenylcarbamoylmethylene)-3-iminoisoindoline,1-[cyano-N-(3'-methylphenyl)carbamoylmethylene]-3-iminoisoindoline,1-[cyano-N-(4'-chlorophenyl)-carbamoylmethylene]-3-iminoisoindoline,1-[cyano-N-(4'-methoxyphenyl)-carbamoylmethylene]-3-iminoisoindoline,1-[cyano-N-(3'-chloro-4'-methylphenyl)-carbamoylmethylene]-3-iminoisoindoline,1-(cyano-p-nitrophenylmethylene)-3-iminoisoindoline,1-(dicyanomethylene)-3-iminoisoindoline, 1-(cyano-1',2',4'-triazolyl-(3')-carbamoylmethylene)-3-iminoisoindoline,1-(cyanothiazoyl-(2')-carbamoylmethylene)-3-iminoisoindoline,1-(cyanobenzimidazolyl-(2')-carbamoylmethylene)-3-iminoisoindoline,1-(cyanobenzothiazolyl)-(2') -carbamoylmethylene)-3-iminoisoindoline,1-[(cyanobenzimidazolyl-2')-methylene]-3-iminoisoindoline,1-[(cyanobenzimidazolyl-2')-methylene]-3-imino-4,5,6,7-tetrachloroisoindoline,1-[(cyanobenzimidazolyl-2')-methylene]-3-imino-5-methoxyisoindoline,1-[(cyanobenzimidazolyl-2')-methylene]-3-imino-6-chloroisoindoline,1-[(1'-phenyl-3'-methyl-5-oxo)-pyrazolidene-4']-3-iminoisoindoline,1-[(cyanobenzimidazolyl-2')methylene]-3-imino-4,7-dithiatetrahydroisoindoline,1-[(cyanobenzimidazolyl-2')-methylene]-3-imino-5,6-dimethyl-4,7-pyraziisoindoline,1-[(1'-methyl-3'-n-butyl)-barbituric acid-5']-3-iminoisoindoline,3-imino-1-sulfobenzoic acid imide, 3-imino-1-sulfo-6-chlorobenzoic acidimide, 3-imino-1-sulfo-5,6-dichlorobenzoic acid imide,3-imino-1-sulfo-4,5,6,7-tetrachlorobenzoic acid imide,3-imino-1-sulfo-4,5,6,7-tetrabromobenzoic acid imide,3-imino-1-sulfo-4,5,6,7-tetrafluorobenzoic acid imide,3-imino-1-sulfo-6-nitrobenzoic acid imide,3-imino-1-sulfo-6-methoxybenzoic acid imide,3-imino-1-sulfo-4,5,7-trichloro-6-methylmercaptobenzoic acid imide,3-imino-1-sulfonaphthoic acid imide, 3-imino-2-methyl-4,5,6,7-tetrachloroisoindolin-1-one, etc. These imino compoundscan be used alone or as a mixture thereof.

The color developer (b) is used in an amount of 20 to 500% by weight,preferably 50 to 300% by weight based on the weight of the dye precursor(a). The imino compound (d) and the isocyanate compound (c) are used inamounts of 1% by weight or more, preferably 5 to 200% by weight,respectively, based on the weight of the dye precursor (a). When theamounts of the imino compound and the isocyanate compound are less than1% by weight, respectively, the effects are insufficient. On the otherhand, when the amounts of the imino compound and the isocyanate compoundare more than 200% by weight, respectively, almost the same effectscannot be expected and the developed color image density is oftenlowered undesirably.

The heat-sensitive recording materials disclosed in U.S. Pat. No.4,521,794 contain a two-component color forming system comprising animino compound and an isocyanate compound, said color forming systemforming a color by a reaction with heating. The resulting developedcolor image is excellent in image storability but does not showabsorption in the near infrared portion when examined by spectralabsorption. On the other hand, the two-component type heat-sensitiverecording material using a fluorene dye precursor disclosed in JapanesePatent Unexamined Publication No. 61-175077 gives a developed colorimage which has an absorption in the near infrared portion but isinsufficient in image storability.

It is a very surprising thing that unexpected excellent properties areobtained when the two-component color forming system of the former andthat of the latter are used together. That is, when the developed colorimage obtained from the fluorene dye precursor and a color developer isexposed to sunlight, the image absorption in the near infrared portion.But when the color forming system comprising an imino compound and anisocyanate compound is used together with the fluorene dye precursor andthe color developer, the resulting image maintains the absorption in thenear infrared portion when exposed to sunlight for a long period oftime. This means that the developed color image obtained from thefluorene dye precursor is remarkably prevented from the deterioration bythe mutual action of the imino compound and the isocyanate compound. Themechanism of this is not known yet, but this is a very specificphenomenon.

In the recording material of this invention, a color forming layer mayfurther contain the following components in addition to the colorforming components depending on purposes.

As a binder, there can be used water-soluble binders such as starches,hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose,gelatin, casein, polyvinyl alcohol, modified polyvinyl alcohol,styrene-maleic anhydride copolymer, ethylene-maleic anhydride copolymer,and the like; and latex type water-soluble binders such asstyrene-butadiene copolymer, acrylonitrile-butadiene copolymer, methylacrylate-butadiene copolymer, and the like.

As a pigment, there can be used diatomaceous earth, talc, kaolin,calcined kaolin, calcium carbonate, magnesium carbonate, titanium oxide,zinc oxide, silicon oxide, aluminum hydroxide, urea-formaldehyde resin,and the like.

In addition, there can also be used a head wear preventing agent or ahead sticking preventing agent such as higher fatty acid metal salts,e.g. zinc stearate, calcium stearate, etc., and waxes, e.g. paraffin,oxidized paraffin, polyethylene, oxidized polyethylene, stearamide,castor wax, etc. in the case of using as a heat-sensitive recordingmaterial; a dispersing agent such as sodium dioctyl sulfosuccinate,etc.; an ultraviolet light absorber such as benzophenones,benzotriazoles, etc.; surface active agents and fluorecent dyes.

As the support for the recording material of this invention, there canbe used paper mainly as well as various non-woven fabrics, plasticfilms, synthetic paper, metal foils, etc., and composite sheets obtainedby combining these materials.

The recording material of this invention can be used for ordinaryfacsimile machines, printers, and the like as well as for special usefor detecting a near infrared light. When high storability of developedcolor images is desired, for example, when used as labels, a protectivelayer may be formed on a color forming layer in order to protect anundeveloped color portion and a developed color image portion fromoutside circumstances. In this case, the protective layer can be formedby using a water-soluble resin, latex, light-curable resin or the likeconventionally used. If necessary, one or more pigments, waterresistance improving agents, water repellents, antifoamers, ultravioletlight absorbers, etc., may be added thereto. Further, in the case ofusing for labels, an adhesive layer may be formed on a rear side of asupport, so that the support can be adhered to a surface of anothersolid material after printed. In this case, a release paper is placed onthe adhesive layer.

The color forming layer may take a single layer structure or amulti-layer structure containing a plurality of layers. In the case ofthe multi-layer structure, one or more intermediate layers may beinterposed among individual layers. The color forming layer can beobtained by making aqueous dispersions of finely ground individual colorforming components, mixing these dispersions with a binder and otheradditives depending on purposes, coating the resulting mixture on asupport, followed by drying. It is also possible to coat each colorforming component on a support, respectively, to form a multi-layerstructure.

This invention is illustrated by way of the following Examples, in whichall percents are by weight unless otherwise specified.

EXAMPLE 1

3,6-Bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylaminophthalide) inan amount of 20 g and 80 g of a 1% polyvinyl alcohol aqueous solutionwere balled milled to form a dispersion. Then, 50 g ofbis(3-allyl-4-hydroxyphenyl)sulfone and 200 g of a 1% polyvinyl alcoholaqueous solution were balled milled to form a dispersion. On the otherhand, 10 g of 1,3-dimino-4,5,6,7-tetrachloroisoindoline and 40 g of a 1%polyvinyl alcohol aqueous solution were balled milled for 24 hours toform a dispersion. Then, 10 g of4,4',4"-triisocyanate-2,5-dimethoxytriphenylamine and 40 g of a 1%polyvinyl alcohol aqueous solution were balled milled for 24 hours toform a dispersion. These dispersions were mixed and added with 250 g ofa 40% calcium carbonate dispersion, 50 g of a 30% zinc stearatedispersion, and 400 g of a 10% polyvinyl alcohol aqueous solution togive a coating liquid with sufficient stirring. The coating liquid wascoated on a base paper having a basis weight of 55 g/m² so as to makethe coating amount 6 g/m² on solids content, and dried to give aheat-sensitive recording material.

EXAMPLE 2

The process of Example 1 was repeated except for using1,4-diisocyanate-2,5-diethoxybenzene in place of4,4',4"-triisocyanate-2,5-dimethoxytriphenylamine to give aheat-sensitive recording material.

EXAMPLE 3

The process of Example 1 was repeated except for using4,4'-thiobisphenol in place of bis(3-allyl-4-hydroxyphenyl)sulfone togive a heat-sensitive recording material.

COMPARATIVE EXAMPLE 1

The process of Example 1 was repeated except for not using1,3-diimino-4,5,6,7-tetrachloroisoindoline and4,4',4"-triisocyanate-2,5-dimethoxytriphenylamine to give aheat-sensitive recording material.

COMPARATIVE EXAMPLE 2

The process of Example 3 was repeated except for not using1,3-diimino-4,5,6,7-tetrachloroisoindoline and4,4',4"-triisocyanate-2,5-dimethoxytriphenylamine to give aheat-sensitive recording material.

EXAMPLE 4

The process of Example 1 was repeated except for using3-dimethylamino-6-diethylaminofluorene-9-spiro-3'-(6'-dibutylaminophthalide)in place of3,6-bis-(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylaminophthalide)to give a heat-sensitive recording material.

COMPARATIVE EXAMPLE 3

The process of Example 1 was repeated except for not using3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylaminophthalide)and bis(3-allyl-4-hydroxyphenyl)sulfone to give a heat-sensitiverecording material.

The heat-sensitive recording materials obtained in Examples 1 to 4 andComparative Examples 1 to 3 were subjected to the following tests:

(1) Developed color density

Using a heat-sensitive paper printing tester (mfd. by MatsushitaElectronic Components Co., Ltd.), printing was carried out underconditions of applied pulse of 3.0 millisec. and applied voltage of16.00 volts. The density of obtained developed color image was measuredby using a Macbeth densitometer RD 918.

(2) Light resistance of image

The developed color image obtained in above (1) was retained inside thewindow glass at a south side and exposed to sunlight for 12 days.Densities before and after the test were measured in the same manner asin above (1) to give the image retention rate by the following equation:##EQU1##

(3) Near infrared light reading-out

Using a bar code printing apparatus (mfd. by Ishida Scales MFG. Co.,Ltd.), printing was carried out at 22.7 volts. The printed matter wassubjected to sunlight exposure in the same manner as described in above(2).

Goodness of reading out of the bar code before and after the test wasexamined by using a light pen for reading bar codes having a wavelengthof about 960 nm (mfd. by Nippon Electric Industry Co., Ltd.).

The results are shown in the following Table

                  TABLE 1                                                         ______________________________________                                                                       Near infrared light                                      Developed  Light     reading-out                                              color      resistance                                                                              Before After                                   Example No.                                                                             density    (%)       test   test                                    ______________________________________                                        Example 1 0.98       67.2      Good   Good                                    Example 2 1.04       70.1      Good   Good                                    Example 3 0.96       68.0      Good   Good                                    Example 4 0.98       67.8      Good   Good                                    Comparative                                                                             1.05       8.0       Good   Poor                                    Example 1                                                                     Comparative                                                                             1.00       8.3       Good   Poor                                    Example 2                                                                     Comparative                                                                             0.90       98.6      Poor   Poor                                    Example 3                                                                     ______________________________________                                    

What is claimed is:
 1. A recording material which comprises a colorforming layer containing a color forming system comprising(A) (a) acolorless or light-colored dye precursor represented by the formula:##STR3## wherein R₁ through R₆ are independently a lower alkyl group,(b) a color developer which can form a color by reacting with the dyeprecursor, and (B) (c) an isocyanate compound having aromaticity, and(d) an imino compound having at least one >C═NH group,said color forminglayer being carried on a support.
 2. A recording material according toclaim 1, wherein the imino compound (d) is a compound of the formula:

    φC═NH                                              (II)

wherein Φ is an aromatic compound residue which can form a conjugatedsystem with the neighboring C═N.
 3. A recording system according toclaim 2, wherein the imino compound (d) is1,3-diimino-4,5,6,7-tetachloroisoindoline.
 4. A recording materialaccording to claim 1, wherein the dye precursor (a) is3,6-bis(dimethylamino)-fluorene-9-spiro-3'-(6'-dimethylaminophthalide)or3-dimethylamino-6-diethylaminofluorene-9-spiro-3'-(6'-dibutylaminophthalide).5. A recording material according to claim 1, wherein the isocyanatecompound (c) is triisocyanate-2,5-dimethoxytriphenylamine or1,4-diisocyanate-2,5-diethoxybenzene.
 6. A recording material accordingto claim 1, wherein the imino compound (d) is a compound of the formula:

    φC═NH                                              (II)

wherein Φ is an aromatic compound residue which can form a conjugatedsystem with the neighboring C═N.
 7. A recording material according toclaim 1, wherein the color developer (b) is used in an amount of 20 to500% by weight, the isocyanate compound (c) is used in an amount of 1%by weight or more, and the imino compound (d) is used in an amount of 1%by weight or more, based on the weight of the dye precursor (a).
 8. Arecording material according to claim 1, wherein a plurality of supportsis used to form a multi-layer structure.
 9. A recording material whichcomprises a color forming layer containing a color forming systemcomprising;(A) (a) a colorless or light-colored dye precursorrepresented by the formula: ##STR4## wherein R₁ through R₆ areindependently a lower alkyl group, (b) a color developer which can forma color by reacting with the dye precursor, with heating selected fromphenol, phenol derivatives, or carboxylic acid derivatives, and (B) (c)an isocyanate compound having aromaticity, and (d) an imino compoundhaving at least one >C═NHgroup, said color forming layer being carriedon a support.
 10. A recording material according to claim 9, wherein thephenol derivative is at least one member selected from the groupconsisting of p-t-butylphenol, p-phenylphenol, 1-naphthol, 2-naphthol,p-hydroxyacetophenone, 2,2'-dihydroxybiphenyl,4,4'-isopropylidenediphenol,4,4'-isopropylidene-bis(2-t-isopropylidenediphenol,4,4'-isopropylidene-bis(2-t-butylphenol),4,4'-isopropylidenebis(2-chlorophenol), 4,4'-cyclohexylidenediphenol,2,2-bis(4-hydroxyphenyl)-butane, 2,2-bis(4-hydroxyphenyl)pentane,2,2-bis(4-hydroxyphenyl)hexane, methyl diphenolacetate,1,7-bis-(4-hydroxyphenylthio)-3,5-dioxiheptane, and novolac phenolresin.
 11. A recording material according to claim 9, wherein thecarboxylic acid derivative is at least one member selected from thegroup consisting of benzoic acid, p-t-butyl benzoate, p-hydroxybenzoicacid, methyl p-hydroxybenzoate, isopropyl p-hydroxybenzoate, benzylp-hydroxybenzoate, lauryl gallate, stearyl gallate, salicylanilide,5-chloro-salicylanilide, a metal salt of 5-t-butyl salicylate, and ametal salt of hydroxynaphthoic acid.